1. Field of the Invention
This invention relates to a titanium oxide film and a production apparatus of a titanium oxide film. More particularly, this invention relates to a titanium oxide film and a production apparatus of a titanium oxide film that will be used suitably for producing a crystalline silicon solar cell. The titanium oxide film of the present invention can be used suitably as an antireflection coating (ARC) of a solar cell.
2. Description of the Related Arts
A solar cell has a p-n junction and an ARC. The method of forming them is described in, for example, Japanese Unexamined Patent Publication No. HEI 8(1996)-085874.
A production process of a solar cell for a module employing a method of simultaneously forming a p-n junction and an ARC according to the prior art is illustrated in FIG. 9.
Ruggedness or trenches (hereinafter simply referred to as the "ruggedness") having a very fine level differences of several to dozens of microns are formed on a surface of a p type crystalline silicon substrate in order to reduce surface reflection on a light incident surface and to prevent a short-circuit current. This ruggedness can be formed, for example, by a texture etching method in which a substrate is etched using a mixed solution of an alcohol and an aqueous sodium hydroxide solution having a concentration of a few percents. A method of forming a large number of trenches by using a dicing device or laser, or dry etching may also by used (S11).
After the ruggedness is thus formed, a Phospho-Titanate Glass (PTG) film containing phosphorus as a dopant element is deposited on the surface of the substrate heated to about 200.degree. C. to about 500.degree. C. by an atmospheric pressure chemical vapor deposition (CVD) method by using titanium alkoxide and a phosphoric acid ester (S12).
Next, the substrate on which the PTG film is formed is heated at about 800.degree. C. to about 1,000.degree. C. in a nitrogen atmosphere. In consequence, phosphorus is diffused from the PTG film into the substrate so that a p-n junction in the substrate and an ARC are formed at the same time.
FIG. 10 shows a refractive index of the PTG film formed on the surface of the silicon substrate and a sheet resistance of the dopant element diffusion layer (n layer) after the film formation, relative to the substrate temperature employed for the formation of the PTG film according to the prior art. This film has a refractive index of about 1.6 to about 2.0. The atmospheric pressure CVD method can form a film having a uniform thickness. The PTG film functions as an ARC by interference. When the PTG film is heated at 900.degree. C. in a nitrogen atmosphere for 30 minutes, the sheet resistance of the n layer is 50 to 500 .OMEGA./.quadrature. (square). A solar cell adapted to a module can be obtained when the sheet resistance is not greater than 100 .OMEGA./.quadrature.(S13).
Next, a titanium oxide film or the like having a higher refractive index than that of the PTG film is uniformly deposited on the PTG film by CVD method or the like. The refractive index of the titanium oxide film can be varied from about 1.8 to about 2.4 by changing the heating temperature of the silicon substrate within the range of 150 to 350.degree. C., as described in Japanese Unexamined Patent Publication No. SHO 62(1987)-104081 (S14).
Next, an aluminum paste is applied to a back surface of the silicon substrate by screen printing method and then fired at about 700.degree. C. to about 800.degree. C. Thus, a back surface electrode made of aluminum is formed and a back surface field layer is formed by diffusing aluminum from the aluminum paste into the back surface of the substrate (S15).
Then, a silver paste is applied to a light incident surface by screen printing method and then fired to give a light incident surface electrode (hereinafter referred to as a grid electrode). Since a glass frit or the like is contained in the silver paste, the light incident surface comes into contact with the dopant element diffusion layer while penetrating through the titanium oxide film and the PTG film (S16).
A continuous atmospheric pressure CVD apparatus that can be used for forming the p-n junction and the reflection preventing film is described in Japanese Unexamined Patent Publication No. HEI 8(1996)-085874, for example. A dispersion head of the continuous atmospheric pressure CVD apparatus is constituted as shown in FIG. 11. An assembly 101 of the head includes a ceiling plate 102, four side plates (a forward side plate is not shown) 103 extending down from the periphery of the ceiling plate 102 and a large number of partition plates 104 interposed between the right and left side plates 103 with predetermined gaps between them. Two gas introduction ports 105 and 106 are provided on the backward side plate 103 between the ceiling plate 102 and the upper ends of the partition plates 104. A cooling plate 107 with a built-in piping for passing air as a coolant is fitted to the outer surface of each side plate 103.
For the film formation, gases containing the starting materials are introduced through the gas introduction ports 105 and 106 into a space between the ceiling plate 102 and the upper ends of the partition plates 104 and mixed together. The resulting mixed gas G is blown downward along the partition plates 104 and supplied to the surface of the silicon substrate 21 that travels below the assembly 101 while being conveyed by a conveyor belt 23. This gas G decomposes on the surface of the silicon substrate 21 and forms a film having a composition that corresponds to the kinds of the starting materials on the surface of the silicon substrate 21. The remaining gas is discharged from an exhaust port 110 to the outside through a gap 109 between the assembly 101 and a cover protuberance 108.
The temperature of the assembly 101 is adjusted to a temperature higher than the dew points of the starting materials but lower than the lower limit of the decomposition temperature of the starting materials by supplying air into the cooling plate 107 at a suitable flow rate.
When a phosphorus diffusion layer (dopant element diffusion layer) having a sheet resistance value of 50 to 100 .OMEGA./.quadrature. is formed by the method of producing a solar cell described in Japanese Unexamined Patent Publication No. HEI 8(1996)-085874 as shown in FIG. 10, the refractive index of the resulting PTG film is from 1.6 to 2.0.
Where the light incident surface electrode is formed by using the silver paste, the loss of a series resistance becomes great due to the increase of the contact resistance. Therefore, a dopant element diffusion layer having a sheet resistance of not greater than about 60 .OMEGA./.quadrature. must be formed to prevent the drop of a fill factor.
In many cases, the crystalline silicon solar cell is assembled in a so-called "super-straight type module". This module comprises a solar cell, a glass and a filler (generally EVA (ethylene vinyl acetate copolymer)) for protecting the light incident surface of the solar cell, a back surface material, a peripheral seal material and a frame member encompassing the periphery. To be assembled in the module, the glass and EVA are positioned on the light incident surface of the solar cell. For this reason, an ARC that has a refractive index different from a diffraction index in the case where the light incident surface of the solar cell keeps direct contact with air is required. Here, the optimum refractive index n of the reflection preventing film is given by n=(n.sub.0.multidot.n.sub.s).sup.1/2 provided that n.sub.s is the refractive index silicon and n.sub.0 is the refractive index of the material of the reflection preventing film. In a wavelength range .lambda.=600 to 1,100 nm, where the sensitivity of the solar cell is high, the refractive index n.sub.s of silicon is from about 3.5 to about 4. When the light incident surface of the solar cell keeps direct contact with air (n.sub.0 =1), the optimum refractive index of the reflection preventing film is 1.8 to 2.0. Where the glass and EVA exist on the light incident surface of the solar cell n.sub.0 =1.4 to 1.5), the optimum refractive index of the reflection preventing film is 2.2 to 2.5.
As can be seen also from FIG. 10, the PTG film (having a refractive index of about 1.6 to about 2.0) is somewhat low to be used as the reflection preventing film of the solar cell used for the module, so that it cannot effectively reduce the surface reflection. Therefore, it has been necessary to uniformly deposit a film having a higher refractive index than that of the PTG film such as a titanium oxide film on the PTG film by CVD method or the like.